Sharif Mobley, a U.S. citizen accused of killing a hospital guard in Yemen, is believed to be a homegrown radical who left this country to make direct contact with al-Qaeda, according to U.S. counterterrorism officials, making him the latest in a string of such suspects.

Mobley, 26, first came to public attention Wednesday, when Yemeni authorities reported that he had grabbed a guard's gun during a medical visit last weekend after being arrested in a sweep of suspected al-Qaeda militants.

Several U.S. officials, all of whom spoke on the condition of anonymity, said Mobley had been in custody in Yemen for at least several weeks before the shooting and had been known to U.S. and Yemeni authorities for a considerable period before that. "He's been a matter of some concern for a while," according to one official.

The officials said FBI investigations had been underway in Delaware, among the places that Mobley had lived, and in New Jersey, where he was born and was once employed as a maintenance worker in nuclear power plants. The Nuclear Regulatory Commission said Friday that he also worked at nuclear plants in Pennsylvania and at Maryland's Calvert Cliffs but that it had no reason to believe he had had access to sensitive material, news services reported.

The Associated Press quoted Mobley's father as saying his son was innocent.

Revelations about Mobley's arrest came amid rising U.S. concern about the radicalization of American Muslims. Recent cases include that of Army Maj. Nidal M. Hasan, who was charged with killing 13 people at Fort Hood, Tex., in November, as well as that of five young men who were arrested in Pakistan and charged with terrorist offenses in December after their parents reported them missing from their Washington area homes.

This week, a 46-year-old Philadelphia woman, a Muslim convert who identified herself online as Jihad Jane, was charged with plotting to kill a Swedish cartoonist who had caricatured the prophet Muhammad.

Many terrorism suspects from the West are said to have frequented jihadist Web sites, and Hasan and others -- including the Nigerian charged with the Christman airline bombing attempt in Detroit -- were in touch online with Anwar al-Aulaqi, a dual U.S.-Yemeni citizen being sought by both governments as a member of al-Qaeda's Yemen affiliate.

Asked whether Mobley had also been in touch with Aulaqi, a U.S. official said, "Everyone has."

Officials said Mobley's mother was of Somali origin. Several Somali Americans are thought to have traveled to Somalia to join the al-Shabab militia. Although senior militia leaders are closely linked to al-Qaeda, many in the rank and file are said by U.S. officials to be fighting for nationalistic reasons confined to Somalia.

But officials said they did not think Mobley had traveled to the region as what one called a "wannabe" soldier in Somalia. He obtained a Yemeni visa ostensibly to study Arabic, the official said, but "went to hook up" with al-Qaeda.

At the State Department on Friday, spokesman P.J. Crowley said that U.S. consular officials in Yemen were attempting to verify Mobley's identity but had not been able to meet with him.

But other officials indicated that U.S. law enforcement at least had access to interrogations of Mobley after his initial arrest. "There would be an interest by the U.S. government to see what he knows, what his experience was like" with al-Qaeda, another official said. His situation, the official said, "may change his thinking, now that he has gone from [being picked up in] a sweep to a murder charge."